a) Field of the Invention
The present invention relates to an exposure measuring apparatus, and more particularly to an exposure measuring apparatus to be used for the exposure control of a camera.
b) Description of the Related Art
The exposure of a camera is generally expressed by a number of 2 raised to some power in order to match the luminosity and broaden the dynamic range. To this end, circuit elements having a logarithmic conversion function are used commonly. A photometric sensor is fabricate, on a semiconductor chip with other devices such as an automatic focus (AF) sensor and various other controlling, measuring, or processing circuits.
As a photometric sensor, a photodiode is generally used. An output of the photodiode is logarithmically converted and measured. An example of a photometric circuit is shown in FIG. 3. A photodiode PD is connected across the input terminals of an operational amplifier.
The relationship between a phtocurrent Ip and an electromotive force V of a photodiode PD is well known and given by: EQU Ip.perspectiveto.exp(qV/kT)-1.perspectiveto.exp(qV/kT).
A voltage across the photodiode is therefore expressed by a logarithm of a current. The current flowing through the photodiode is proportional to the intensity of incident light so that an output of the operational amplifier is also proportional to the logarithm of the intensity of incident light.
The photocurrent Ip is typically expressed as a number including a term of 2 raised to some power such as: EQU Ip.perspectiveto.Io2.sup.qv/kt.
An output voltage V of the photodiode is therefore given by: ##EQU1## where q is an electric charge elementary quantity, k is the Boltzmann constant, T is an absolute temperature, and Vo is an offset voltage.
In general, the photocurrent Ip of a photodiode substantially independent of temperature changes (and hardly changes with temperature), except that the photocurrent is affected by a temperature change in the band gap of semiconductor. However, as is apparent from the above equation, the output voltage of a photodiode changes greatly in proportion to temperature. As the temperature changes, the output of the operational amplifier does not represent the logarithm of the intensity of incident light.
From this reason, the conventional exposure sensor having the circuit shown in FIG. 3 requires atemperature compensating circuit in addition to the photometric circuit.
A temperature compensating circuit requires a complicated circuit arrangement, resulting in a large chip area. It is therefore difficult to integrate these circuits as well as other devices such as an AF sensor on the same chip.
There arise therefore problems such as an increased number of chips, a high cost, and an obstacle against compactness.